There are many uses in industry for valves in which the pressure differential across the main closure member is controlled in one way or another to aid in the closing and/or opening of the valve. An example is found in so-called balanced valves in which the main closure member is partially positioned within an auxiliary chamber which is pressurized with fluid under substantially the same pressure as the fluid in the line so that the closure member can be moved to close the valve without moving it across a large pressure differential.
Such valves are usually closed by large springs or some other suitable source of energy.
There are several disadvantages to such a design. For one, if the valve is large, a large force must be made available to close the valve even if it is a balanced valve. Further, the availability of pressurized fluid to aid in the closing of a valve of the balanced design is a problem when the line in which the valve is placed fails on a side thereof from which fluid is taken to pressurize the auxiliary chamber.
One way to provide a closing force is to design the valve so that fluid under pressure can be provided for the auxiliary chamber from either side of the valve. This requires an additional system of control valves which is expensive and, because of the additional critical components, unreliable unless maintained rigorously.
In large valves, the use of line pressure as a source of energy to supply a closing force for a valve is necessitated by the impracticability of providing power sources separate from the line pressure, which can exert a force greater than that exerted by the fluid in the line. Thus, even with the disadvantages pointed out above, line pressure has been used to exert the energy to close large valves.